There is an increasing need in the pharamaceutical, chemical and related industries for a source of highly purified water which is available in large quantities, yet low in cost. Ultrafiltration has proven to be effective in meeting the needs of these industries. Ultrafiltration has an advantage over other filtration systems in that ultrafilters do not directly trap excluded particles; hence, the filtration membrane does not rapidly lose permeability.
In operation, ultrafiltration is a process in which a pressurized solution is caused to flow across a membrane surface. The membrane is designed so that water and species smaller in size than the rejection dimensions of the membrane will pass through the membrane, while larger species will be rejected at the membrane surface and pass downstream to be eliminated in a rejection flow.
A problem encountered in the ultrafiltration of large volumes of water is the build-up of rejected species which do not traverse the membrane. Such rejected species, though not trapped within the membrane, accumulate upon the ultrafiltration membrane surface. This phenomenon is called concentration polarization. Within an ultrafiltration system there is an impressed pressure differential across the membrane. Water and other very small molecules pass through the ultrafiltration membrane. Solutes larger than the rejection size of the membrane travel to the membrane surface but do not traverse the membrane surface. At the ultrafiltration surface such species accumulate. These species are removed from the surface only by back diffusion into the bulk flow. Since the water flux of ultrafiltration membranes is high, the convective transport rate is initially much higher than the diffusive back transport rate. A concentration of solute therefore builds up at the membrane surface until the solutes precipitate and form a gel. The thickness of this gel layer will increase until its hydraulic resistance to water transport reduces the water flux to an equilibrium value. At equilibrium the convection transport equals the diffusive transport and ultrafiltration is inhibited. Once concentration polarization is in control, increasing the pressure of the stream will not increase the flux since the higher pressure will cause a thicker layer of gel and hence greater resistance.
Typically, in large volume water purification, to avoid the rapid build-up of flow-impeding film on the ultrafiltration membrane, only highly filtered water is used as the ultrafiltration affluent. To achieve this end, raw water is exposed sequentially to filtration through carbon columns, cation resin columns, and anion resin columns; and, recently the filtration art has included a filtration through a large-pore macroreticular anion exchange resin column. This last resin has the advantage of removing most of the bacteria, virus, pyrogen, and colloid contamination from the water prior to ultrafiltration.
The typical preultrafiltration steps thus described requires that the total flow of the system be presented to the large-pore macroreticular anion exchange resin. Such filtration requires a large amount of large-pore macroreticular anion exchange resin due to the flow volume and the relative lack of concentration of bacteria, viri, pyrogens, and colloids. Furthermore, the system described then discards as waste that portion of ultrafiltration flow which does not traverse the ultrafiltration membrane.